There are many applications in which it is desirable to track or locate objects or people, such as: tracking athletes for training or providing event information in real time; tracking emergency services or military personnel in buildings and urban environments; tracking staff, patients, and equipment in hospitals and nursing homes; and tracking staff and equipment in industrial, hazardous or mining environments for safety and automation.
The Global Positioning System (GPS) has been widely used for tracking across a wide range of applications, however there are many situations in which GPS cannot be used due to cost, accuracy, and reliability and availability of GPS signals. For example GPS cannot usually be used for tracking within large buildings or in underground mines. A solution in such cases is to deploy a local positioning system consisting of an infrastructure of anchor nodes at known locations and mobile nodes attached to the objects to track. The location of the mobile nodes can be determined in two or three dimensions through an exchange of radio signals between the mobile nodes and the anchor nodes.
A problem with local positioning systems, particularly in environments with constraints on signal propagation such as buildings and mines, is that the required anchor node density for full site coverage is not practical. Furthermore, the radio communications systems in such environments are frequently ad hoc or mesh networks. The requirements on the anchor nodes can be reduced by using “cooperative localisation”. In cooperative localisation, signals are not only exchanged between anchors and mobile nodes, but they are also exchanged between mobile nodes, hence nodes cooperate to determine their location using the network.
Cooperative localisation usually uses distance measurements between communicating node pairs, the distance being estimated from the exchanged radio signals and typically be based on time of arrival or signal strength.
A simple form of cooperative localisation is iterative multilateration. Multilateration is used to determine the locations of mobile nodes with sufficient distance measurements to anchor nodes. The computed mobile nodes are then treated as anchor nodes and this procedure is iterated until no more mobile nodes can be located. Two problems exist with this approach: the error performance is poor, and in many network topologies the location of many of the mobile nodes cannot be estimated by this algorithm.
A need exists, therefore, to provide improved location tracking.